1. Field of the Invention
The present invention relates to a system of soldering and particularly to an improved nozzle assembly used to deposit solder in a precise manner, as where solder is deposited onto solder wettable contact pads utilized in the semiconductor industry.
2. Description of the Prior Art
Solder distribution onto mounting pads for surface mount boards of the type used in the semiconductor industry has generally been accomplished in the prior art through the use of screening processes. In these techniques, art work and screens are fabricated having the solder deposition pattern. Then, a precision alignment process is carried out wherein the solder is screened onto the surface mount pads. The solder paste used for this process requires a substantial cure time and a substantial bake time. Various problems result due to the complexity of the alignment process making the prior art technique relatively time consuming
The prior art technique utilizing screening is further complicated by a requirement that the pattern mixes very fine lead pitch and width surface mount pads along with standard surface mount parts. For example, in the case of tape automated bonding, the pitches vary from about 4 to 20 mils while, the standard surface mount parts have pitches in the range from about 20 to 50 mils. Due to the precision nature of the operation, it is common to utilize separate screening steps, one for the very fine lead pitch and width surface mount parts and second for the standard surface mount parts. There is the possibility of damaging the solder deposited in a previous step when multiple screening operations are carried out. Additionally, screening fine line solder represents a problem because the solder paste tends to stick in the openings of the screen as the openings get progressively narrower.
The prior art screening systems also present problems due to the difficulty of performing rework operations. Once a defective part has been removed, there is no commonly available technique for replacing the solder on the board site. While techniques exist to remove the solder after the part has been removed, replenishing the isolated site by screening is not generally feasible.
In the copending application of Raymond E. Prime and Arthur L. Leersen, entitled "Solder Placement Nozzle Assembly", filed concurrently herewith and assigned to the assignee of the present invention, a device is shown which is usable either manually or under robotic control to deposit solder onto fine line pitch and width surfaces in precise fashion. That device is capable of depositing solder on pads of surface mount board without the requirement for artwork or solder screens.
In order to provide a nozzle assembly which could be intricately machined, which was lightweight and durable, and which was of a non-wettable, non-conductive design, a polyimide material was utilized. Although providing the foregoing advantages, this material was subject to degradation due to the soldering operating temperatures (650.degree.-800.degree. F.) and presence of oxygen in the air surrounding the assembly. The result was unacceptable degradation of the heated tip of the soldering iron, the housing material of the nozzle assembly itself, and of the flux in the cored solder being applied by the nozzle assembly.
Another problem encountered during the operation of the nozzle assembly concerned the environment around the surface of the solder ball being applied by the solder placement nozzle. In order to prevent the formation of oxide skins on the surface of the molten solder it was initially necessary to apply generous amounts of flux to the solder site. Large amounts of flux dispensed on the solder sites produced cleanability problems, however, and sometimes contributed to solder "bridging" defects.
The present invention, accordingly, has as its object to provide a flow of a relatively cool, inert bleed gas past the heated metal tip of the soldering iron to reduce the rate and amount of degradation of the coating material on the soldering iron, as well as the rate of degradation of the fabricated nozzle housing material and the flux from the cored solder.
Another object of the invention is to exclude oxygen in the surrounding air from the interior of the nozzle assembly, by the provision of a suitable bleed gas, to thereby prevent a reaction with the material of the housing, the heated metal tip and the flux.
Another object of the invention is to provide a flow of inert cover gas to the solder application site, during soldering operations, to inhibit the formation of oxide skins on the surface of the molten solder being applied to the target substrate.
Another object of the invention is to lower the incidence of solder bridging due to polymerizing and charring of the flux being applied.
Another object of the invention is to reduce the amount of flux required for successful solder operations.